1. Field of the Invention
The present invention relates to a cleaning device and an image forming apparatus, more specifically to a cleaning configuration for cleaning a spherical toner such as a toner obtained by a polymerization.
2. Description of the Related Art
As is generally known, in an image forming apparatus such as a copier, printer or a printing machine, a photoconductor as a latent electrostatic image bearing member is charged uniformly in a charging step, and a latent electrostatic image is formed thereon by exposing it to light corresponding to image data.
The latent electrostatic image formed on the photoconductor is visualized by a toner supplied from a developing device, and then the visualized image is transferred by a transferring unit onto a recording medium such as paper or an intermediate transfer member. A transfer onto a recording medium is mainly carried out to form a monochrome image, and a transfer onto an intermediate transfer member is carried out to form a multicolored image such as a full-color image. A multicolored image can be formed in two manners. Firstly, an image of each color is successively transferred onto the intermediate transfer member and the obtained superimposed image is transferred together onto a recording medium. Secondly, when a belt is used as an intermediate transfer member, a recording medium is adsorbed and retained on the belt, and the belt is moved through image forming stations at each of which each specific color is transferred to the recording medium, resulting in a superimposed images. In either mode, the final output of copy is a toner image transferred onto the recording medium.
The image bearing member includes not only the photoconductor but also an intermediate transfer member to which each image formed in each image forming station corresponding to each color is transferred.
When images each having different colors are transferred successively to the intermediate transfer member, the intermediate transfer member is subjected to collective transfer of the images of different colors, followed by cleaning of remaining toner containing a toner not transferred. The purpose of the cleaning is to prevent occurrence of background smear due to the transfer of the remaining toner.
Conventionally, a blade cleaning method is well known for use in the above cleaning treatment.
In the blade cleaning method, a blade is brought into contact with the object to be cleaned, intercepts the remaining toner moving toward the blade, and sweeps the toner from the object to be cleaned.
Separately, in recent years, in accordance with a demand for high resolution and high image quality, a toner produced by a polymerization method has been used, in place of the conventionally used toner produced by a pulverization method, since according to the polymerization method, particle size can be uniformly confined to be within a small range and a particle of high sphericity can be obtained.
The toner produced by a polymerization method is, because of its high sphericity, advantageous in that an efficiency of transfer is improved and an amount of a toner discarded as a non-transferred toner is minimized, and therefore has been frequently used. Next, the reason therefor will be explained in more detail.
In the developing step, the toner is provided to the electrostatic latent image borne on the image bearing member such as a photoconductor, under the condition that the toner is given a development bias.
The forces that act on the surface of the image bearing member when the toner adheres on the image bearing member due to the latent electric potential and the development bias on the image bearing member is mirror force and Van der Waals' force. The mirror force depends largely on the amount of electric charge and the distance.
The pulverized toner obtained by a conventional pulverization method has a concavo-convex surface, whose convex portions are intensively charged by frictional charging. In contrast, since a polymerized toner produced by the polymerization method has a spherical shape or close to a spherical shape, the surface thereof is uniformly charged. Moreover, in the pulverized toner, toner particles are in contact with each other at convex portions, suggesting that much charge is concentrated in a very close region so that mirror force increases. If however the toner has a spherical shape as in a polymerized toner, each toner is in contact with each other at a point, and a charge amount in the close region is small, and the mirror force is weaker than that of the pulverized toner.
Since the most pulverized toner is in contact with each other at the convex portions, Van der Waals' force is very large. In contrast, since the polymerized toner has a spherical shape and contacts with each other at a point, the Van der Waals' force of the polymerized toner is smaller than that of the pulverized toner.
Thus, in terms of contact force, due to its spherical shape, the polymerized toner has small attachment force such as small mirror force, and Van der Waals' force to a photoconductor, so that it reduces an amount of the remaining toner after transfer, reducing consumption of the toner, and thus it is economically advantageous.
However, when cleaning the remaining toner, the polymerized toner, which has a small particle diameter and spherical shape, tends to pass through the gap between the blade and the surface of the image bearing member. Therefore, in order to remove the polymerized toner on the image bearing member, it is required to strongly press the blade to the surface of the image bearing member enough to intercept the toner. However, strongly pressing the image bearing member by the blade accelerates the abrasion of the blade and the image bearing member. Moreover, in order to press the blade to the image bearing member, a torque of the motor for driving the image bearing member has to be large, correspondingly.
As a cleaning method for cleaning a small-sized, peripheral toner without causing damages to the image bearing member, a method using a electrostatic brush roller which attracts a toner by electrostatic force has been considered (see Japanese Patent Application Laid-Open (JP-A) No. 2007-121568, Japanese Utility Model Application Laid-Open (JP-U) No. 64-36867, Japanese Patent (JP-B) Nos. 3524423 and 3900283).
JP-A No. 2007-121568 discloses that two brush rollers are disposed in contact with an image bearing member in a direction of movement of the image bearing member, and bias voltage of different polarities are applied to the two brush rollers so that the remaining toner is collected by unifying the polarity of the remaining toner.
JP-U No. 64-36867 discloses, similar to the JP-A No. 2007-121568, that two brush rollers are disposed each of which has different hair density, so that the two brush roller can play complementary roles in the efficiency of scraping the toner.
JP-B No. 3524423 discloses that a charging unit having a charging roller for applying a bias voltage of the polarity reverse to the remaining toner on an image bearing member such as an intermediate transfer belt and the charging unit is mildly contacted to the image bearing member, before the image bearing member enters a cleaning device in the image forming process section.
JP-B No. 3900283 discloses that in the traveling direction of the image bearing member that bears the remaining toner on the surface thereof, a charging unit using a scraper or the like capable of applying bias voltage of the same polarity as that of the toner is disposed in contact with the image bearing member in the upstream side; and a brush to which a bias voltage of reverse polarity to the charging unit is applied is disposed in contact with the image bearing member in the downstream side in order to scrape the remaining toner and to unify the polarity of the toner. Thereafter, the remaining toner which has not been scraped by the brush and whose polarity is unified is scraped by applying a bias voltage reverse to the polarity of the toner.